Unfilled chemically cross-linked polyethylene has long been recognized by the power cable industry as an insulation material characterized by very low dielectric losses, high dielectric strength, and excellent physical properties. Its most undesirable properties are relatively high hardness and minimum flexibility. Installation costs increase substantially due to the longer time involved making splices and terminations. Compositions based upon ethylene-propylene copolymers and terpolymers and other elastomers have found application in power cables because of their inherently greater flexibility and ease of installation, substantially reducing those costs.
Ethylene-propylene rubbers have been amorphous materials and required reinforcement from added fillers such as hard calcined clays to achieve their necessary physical strength to be used in insulation. Polyethylene, being crystalline in structure, does not require additional reinforcement, therefore unfilled compositions are practical and are used in most power cable applications above 2 kV rating. An unfilled polymer system provides the highest degree of electrical properties, and conversely, the addition of fillers detracts from the inherently good electrical properties of ethylene-propylene rubbers in proportion to the amount of filler employed.
In my invention, both the highest degree of electrical characteristics of an unfilled polymer system and the inherent flexibility of rubber have been combined into a composition suited to power cable applications which include both low and high voltage varieties. This has been accomplished by physically combining polyethylene and an ethylene propylene copolymer or ethylene propylene terpolymer together with a suitable antioxidant and a peroxide curing agent to bring about a cross-linked composition. The ratios in the polymer system can be varied to provide more or less flexibility as desired without appreciably altering other physical properties and without significant changes in electrical characteristics. The resultant compounds are inherently tough, flexible and of the highest degree of electrical properties comparable to those of a typical unfilled chemically cross-linked polyethylene. The cross-linking itself can also be achieved in the absence of chemical cross-linking by irradiation curing.
Based upon limited data available at this time, the life span of the compounds included in this invention will exceed the normal life span of typical unfilled chemically cross-linked polyethylene compounds. Other properties such as low temperature flexibility, weatherability, resistance to deformation and mechanical damage are at least the equivalent of cross-linked polyethylene and typical ethylene propylene copolymers.
A study of a-c dielectric strength has indicated that the new compositions exceed ethylene propylene rubber formulations by a marked degree and slightly surpass the breakdown voltage achieved by the unfilled cross-linked polyethylene.
This invention includes a novel method of mixing the components of the composition. Raw polyethylene and ethylene propylene copolymer, preferably with 15% crystallinity, are purchased in the form of pellets. These original pellets remain intact throughout the entire pre-blending cycle which both intermixes the polymers themselves and provides for the addition of the anti-oxidant and peroxide curing agent by absorption through the pellet surfaces. This is a modification of the patented Furukawa (U.S. Pat. No. 3,455,752) mixing process for cross-linkable polyethylene. Modifications in the mixing are necessary due to the dual polymer system involved. Final mixing is achieved by the screw in the barrel of the extruder which homogenizes the ingredients of the polymer system to form the insulation before extruding it on the final cable product. At this stage, the polymers are fluxed into each other forming a complete matrix, and the additives are uniformly dispersed in the polymer blend. This is a distinct difference from the original Furukawa process which does not require two polymers to be fluxed in the final extrusion operation.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.